Which of the following molecules is paramagnetic according to Molecular Orbital Theory?

The correct option is O2.

Explanation

According to Molecular Orbital Theory (MOT), the magnetic nature of a molecule is determined by the distribution of electrons in its molecular orbitals. A molecule is paramagnetic if it possesses one or more unpaired electrons. If all electrons are paired, the molecule is diamagnetic.

Analysis of Options

• (a) O₂ (Oxygen): The oxygen molecule has 16 electrons. Based on the energy level diagram for molecules with more than 14 electrons, the electronic configuration is:
  σ1s² σ1s² σ2s² σ2s² σ2p_z² (π2p_x² = π2p_y²) (π2p_x¹ = π2p_y¹).
  The presence of two unpaired electrons in the degenerate π (antibonding) orbitals makes O₂ paramagnetic.
• (b) N₂ (Nitrogen): The nitrogen molecule has 14 electrons. Its configuration is:
  σ1s² σ1s² σ2s² σ2s² (π2p_x² = π2p_y²) σ2p_z².
  All electrons are paired, making it diamagnetic.
• (c) F₂ (Fluorine): The fluorine molecule has 18 electrons. Its configuration fills the π orbitals completely:
  ... (π2p_x² = π2p_y²).
  All electrons are paired, making it diamagnetic.
• (d) Ne₂ (Neon): Theoretically possessing 20 electrons, the bond order is zero (10 bonding electrons minus 10 antibonding electrons), meaning the molecule does not exist stably. However, theoretically, all electrons would be paired.

Key Takeaway: Oxygen (O₂) is paramagnetic due to the presence of two unpaired electrons in its antibonding π molecular orbitals, a feature uniquely explained by Molecular Orbital Theory.